1. Field of the Invention
The present invention is related to a method and to an apparatus for making structural reinforcement preforms for resin transfer molding (RTM) and reaction injection molding (SRIM) processes for structural composites. In the present transfer molding or injection step, the previously formed preform is subject to a deformable plastic material filling the interstices between the fibers of the preform and the plastic material is hardened to form a structural composite comprised of the injected plastic in the interstices and the fibers.
2. Description of the Prior Art
In making directed fiber preforms, it has heretofore been the practice to spray chopped fibers with a binder resin onto a form that has air pulled therethrough to locate and hold the fibers. The form with the fibers and the binder resin is then rotated into a hot air plenum chamber, dried and/or cured to set the binder resin. In addition, a great deal of storage space is required for the preforms for drying and curing. In making thermoformed preforms, it has heretofore been the practice to use a continuous strand fiber mat that has been previously coated, by the fiber manufacturer, with a thermoplastic binder. The thermoformable mat is supplied in a roll form whereby it is unrolled into flat sheets of varied layer thicknesses and clamped into a holding frame at the edges. The frame network is then positioned in an oven chamber containing radiant heaters which slowly heat the reinforcement mat and thermoplastic binder from both sides. Upon heating, the thermoplastic binder softens, and, while soft, the frame network is quickly transferred onto a cold mold. The mold closes via a press forcing the reinforcement mat into the shape of a part. Upon cooling, the thermoplastic binder stiffens and thus holds the thermoformable mat in its new shape. The processes described are slow, require a great deal of space and require a large amount of energy. In practicing the present invention, the preforms can stay in the manufacturing location and leave only when they are cured. The present invention is more energy efficient in that the binder resin only is heated using the energy for curing. The reinforcement will not be heated, nor will the forms. There is therefore no requirement of large rooms with constantly operating ovens. The process is extremely fast with the cure range being in seconds, rather than hours. The process is environmentally safer in that there are few volatiles required and none are driven off as in a system requiring heated ovens.
In conventional RIM/SRIM process applications for structural components, fiber layer thickness across the entire preform is increased to meet the strength requirements of one area which results in unnecessary use of material and increased thickness and weight. Also, neither the directed fiber process nor the process using thermoformable mat allows a designer to add ribs or closed sections to maximize design properties.
Thermoforming and directed fiber processes are slow, cumbersome and wasteful of energy and material.
It is known in the art to apply electromagnetic energy to affect elements in processes, such as the use of ultraviolet radiation and microwave radiation.
As a specific example, it is well known to employ microwaves for generating heat, as in conventional microwave ovens. Many applications for microwave heating have been developed including U.S. Pat. No. 3,597,567, U.S. Pat. No. 2,560,903 and U.S. Pat. No. 3,277,580 for drying paper, textiles, veneers, foods, pharmaceuticals and the like. Such techniques may be employed in practicing the present invention and are incorporated herein by reference. Air flow has also been incorporated in such systems for removing moisture created as steam in the drying process.